It's an infantry regiment in which this happens. Basically the port starts at one end, passes to the left and you pour your partner's glass, but it must NOT touch the table until everyone's glass is full.

I just think there are some strange little traditions that probably date back ions!

Best guess (no real evidence to support it; luckily my regiment are pretty relaxed about most mess things):

The port has to go one way so that everyone gets a go(if it gest passed both ways all the decanters might end up in the middle, next to the fat drunken bloke who will hog it all for himself). I suspect that the Navy may have picked left as easy to remember (Port being left in matelot - why the hell is that? Now there's a question).

I also suspect that not putting it on the table may have been expedient, rather than a custom, in the wardroom of a ship riding out heavy weather. Infantry units that served as marines may have picked up the custom and hung onto it. You could blow this theory out of the water by checking to see if your regiment ever had a spell at sea.

Port not touching the table is a custom in many Messes although it seem more common in infantry Messes. It is done to ensure camaraderie. The loyal toast should not be conducted until all have a glass of port/Madeira (or water only allowed substitute for those who don't drink). The PMC can see the place of the port decanter as it is passes from person to person and if it rests on the table he know all glasses are full and can continue.

In our Mess we don't have that tradition, but no two port decanters are allowed to catch each other as they are passed around the table, a heft fine lands on anyone caught. So fill your glass quick and pass it on

Passing to the left has arisen from etiquette associated with posh dinner parties...............

Seating plan at such a party: Guest of Honour was always seated to the right of the host.

Port decanter would be placed in front of the host and he would pour the drink for the chief guest, on his right. Thereafter everyone else pours their own.

This means that the host, having next poured himself a glass needs to send it around the table. The guy to his right already has some, and rather than stretch across him it makes sense to pass it to the left instead. I imagine this is where the next most important guest would be too.....and so it carries on clockwise.

Voila!

The only Mess I have been in where the port doesn't touch the table is an RAF one - and the claim there (not sure on the truth of it) was that it represented the 'flying' aspect of the RAF!!!

I believe that in Navy wardrooms the decanter is not meant to leave the table and port is poured by tilting decanter with base firmly on the ground. In one wardroom I know it is forbidden to pour your own port - anyone caught doing so is fined........ not sure if this true across the Navy.